Optical transmitting and receiving device and the manufacturing method

ABSTRACT

An insulation film (silicon dioxide film) is laminated on a platform substrate of Si, etc., and a transmitting unit wiring pattern and a receiving unit wiring pattern are provided on the insulation film. Although a base intruded in a convex shape is provided at the bottom of a light emitting device (LED) of the platform substrate, this base is not provided at the bottom of a light receiving device, and the insulation film under the light receiving device becomes thick. A groove is formed in a waveguide, and a WDM filter is mounted. A diamond or SiC layer can also be provided beneath the insulation film to improve the insulation resistance, and a conduction layer is provided beneath the diamond or SiC layer. Electrical crosstalk is suppressed by grounding the conduction layer. Optical crosstalk is improved by locating the LED at the entrance/exit of the WDM filter and the light receiving device on the opposite side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical transmitting and receivingdevice incorporating a light emitting device (LED) and a light receivingdevice into a substrate.

2. Description of the Related Art

Lately, although communication networks using optical fibers are beingput into practice, so far only trunk route communication networks havebeen formed using optical fibers, and channels laid from trunk routenetworks to subscribers, that is, subscriber channels, still remain aselectrical circuits. To further spread the use of optical communicationnetworks, it is desired that all the subscriber channels also consist ofoptical channels. To meet this expectation a variety of research anddevelopment is actively being carried out.

The cost reduction of optical devices including a WDM filter constitutesa very important factor in the construction of an optical subscribersystem using a wavelength multiplex technology called an “ATM-PON(Asynchronous Transfer Mode-Passive Optical Network). To reduce thecost, the mounting of a small number of compact optical devices isindispensable, and a device form in which an LED, a light receivingdevice and a WDM filter are hybrid-mounted on a substrate is expected tobe developed. Furthermore, since the transmitting unit and receivingunit of a optical module operate asychronously in the ATM-PON system, itis necessary that the crosstalk between transmission and reception inthe module is sufficiently small.

FIG. 1 explains the configuration of a conventional optical transmittingand receiving device.

As shown in FIG. 1, in the conventional optical device an LED 1004, alight receiving device 1005 and a WDM filter 1002 are encapsulated asdiscrete devices, and the devices are connected to an optical networkand with each other using optical fibers 1001 and 1003, respectively. Inthe optical module with this configuration the CAN packages of the LED1004 and the light receiving device 1005 are utilized as electrostaticshielding to suppress the crosstalk between transmission and receptionsignals.

Recently, although a miniature optical transmitting and receiving devicehybrid-mounting an LED, a light receiving device and a WDM filter on awaveguide substrate is being developed, the application of this opticaldevice is limited to a TCM (Time-Compression Multiplexing) transmissionsystem for time-dividing transmitting time and receiving time. This isbecause the crosstalk from the transmitting unit to the receiving unitis difficult to suppress. In the transmitting unit, several tens ofmilliamperes of current are required to drive the LED, whereas thereceiving unit requires only very little current, in the order of amicroampere or less. For this reason, the current in the receiving unitis required to be in the order of 10 to 100 nA because of the crosstalkfrom the transmitting unit.

FIG. 2 explains how crosstalk is generated between an LED and a lightreceiving device in a hybrid-mounted optical transmitting and receivingdevice.

To simplify the description, only the minimum necessary componentelements are shown in the diagram.

In an optical transmitting and receiving device hybrid-mounting an LED(laser diode: LD) 1100 and a light receiving device (photodiode: PD)1101, a silicon dioxide film (SiO₂) 1104 is formed on a silicon (Si)substrate 1106, and on the silicon dioxide film electrodes 1102 and 1103are formed. Then, the electrodes 1102 and 1103 are connected to the LD1100 and PD 1101, respectively. A metallic film (not shown in thediagram) is provided to ground the back of the substrate 1106. Althoughthe silicon dioxide film 1104 is provided so that current may not flowin the electrode 1103 of the PD 1101 due to the voltage generated by theelectrode 1102, current leaks to the substrate 1106 by the effect ofalternating voltage applied to LD 1100 since the insulation function ofthe silicon dioxide film 1104 is not complete and the silicon dioxidefilm 1104 itself has its own capacitance. At this moment, although muchof the current flows out of the substrate 1106 since the back of thesubstrate 1106 is grounded, part of the current reaches the electrode1103 through the inside of the substrate 1106. Although the currentreaching the electrode 1103 is small, significant noise appears on thesignal generated by the PD 1101 due to the current reaching theelectrode 1103 through the substrate 1106, since there is a significantdifference between the current for driving the LD 1100 and the currentgenerated by the PD 1101, as described before. Accordingly, theperformance of the PD 1101 in detecting optical signals from thereceived light beams becomes lower because of this generated current.

In this way, as a result of the conventional configuration, significantcrosstalk is generated between the transmitting side and receiving sidethrough the substrate 1106.

The silicon dioxide film 1105 is a heat-oxidized film generated duringthe substrate processing, and if the back of the substrate 1106 is leftunprocessed, the thickness of this silicon dioxide film 1105 will growto approximately 2 μm.

To realize a miniature optical device for an ATM-PON, it is necessary tohybrid-mount an LED, a light receiving device and a WDM filter on thesame substrate, and to reduce the crosstalk between the transmittingside and the receiving side as described before. The crosstalk betweenthe transmitting side and the receiving side is roughly classified intotwo groups; crosstalk due to an optical cause such as stray light, etc.,and crosstalk due to an electrical cause such as free capacitance, etc.

As described before, the electrical cause is generated by a currentflowing between the transmitting side and receiving side through thesubstrate, and this is a serious problem.

The stray light, etc. is generated by light beams emitted from the LDleaking out from an optical waveguide and generating a mode spreadingover all the substrate. Accordingly, when the PD receives such straylight, it becomes impossible to accurately receive optical signals.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a miniature opticaldevice in which transmission and reception can be simultaneouslyoperated, by reducing the crosstalk between a hybrid-mounted LED andlight receiving device.

The optical transmitting and receiving device of the present inventioncomprises a conduction layer formed on all or a part of the surface of asubstrate, an insulation layer formed at least at the bottom of an LEDmounting portion and a light receiving device mounting portion, anoptical waveguide formed on the surface of the insulation layer,electric wiring patterns formed on the surface of the insulation layer,and an LED and a light receiving device connected to the electric wiringpatterns so as to be optically coupled with the optical waveguide. Theoptical transmitting and receiving device is characterized in that theabove-mentioned conduction layer is made electrically connectable to aconstant potential portion.

The manufacturing method of the optical transmitting and receivingdevice of the present invention comprises the steps of forming aconduction layer by doping an impurity on the surface of the substrate,laminating an insulation layer on the surface of the conduction layer,providing an optical waveguide on the insulation layer and mounting anLED and a light receiving device.

The optical transmitting and receiving device in another aspect of thepresent invention is characterized in that in an optical transmittingand receiving device hybrid-mounting at least an LED and a lightreceiving device on the same substrate through the insulation layer, aconduction layer is located at least at the bottom of theabove-mentioned LED and the above-mentioned light receiving device, andbetween the above-mentioned substrate and the above-mentioned insulationlayer, and the conduction layer can be electrically connected to aconstant potential portion.

The optical transmitting and receiving device in another aspect of thepresent invention is characterized in that in an optical transmittingand receiving device hybrid-mounting at least an LED and a lightreceiving device on the same substrate through the insulation layer, theabove-mentioned substrate is a semiconductor substrate of one (p or ntype) conduction type, and a semiconductor layer of one (n or p type)conduction type the reverse of the above-mentioned (p or n type)conduction type, forming a pn junction together with the above-mentionedsemiconductor substrate, and is located at least at the bottom of theabove-mentioned LED and the above-mentioned light receiving device andbetween the above-mentioned semiconductor substrate and theabove-mentioned insulation layer, and the voltage applied between theabove-mentioned LED and a light receiving device, and the back of theabove-mentioned semiconductor substrate is biased the reverse of theabove-mentioned pn junction.

The manufacturing method of the optical transmitting and receivingdevice in another aspect of the present invention is characterized inthat in the manufacturing method of an optical transmitting andreceiving device hybrid-mounting at least an LED and a light receivingdevice on the same substrate through the insulation layer, comprises aprocess for forming a conduction layer electrically connectable to theconstant potential portion on the surface layer portion of theabove-mentioned substrate and at the bottom of at least theabove-mentioned LED mounting portion and the above-mentioned lightreceiving device mounting portion, prior to the formation of theabove-mentioned insulation layer.

The manufacturing method of the optical transmitting and receivingdevice in another aspect of the present invention is characterized inthat in the manufacturing method of an optical transmitting andreceiving device hybrid-mounting at least an LED and a light receivingdevice on the same substrate through the insulation layer, comprises aprocess for forming a conduction layer of one (n or p type) conductiontype the reverse of the above-mentioned (p or n type) conduction type,forming a pn junction together with the above-mentioned semiconductorsubstrate, on the surface layer portion of the above-mentioned substrateand at the bottom of a portion mounting at least the above-mentioned LEDand the above-mentioned light receiving device, prior to the formationof the above-mentioned insulation layer using a semiconductor substrateof one (p or n type) conduction type for the above-mentioned substrate.

The platform of the optical transmitting and receiving device of thepresent invention comprises a conduction layer formed on all or a partof the surface of the substrate, an insulation layer formed at thebottom of portions mounting at least an LED and a light receivingdevice, an optical waveguide formed on the surface of the insulationlayer, and an electric wiring pattern formed on the surface of theinsulation layer, and is characterized in that the above-mentionedconduction layer can be electrically connected to a constant potentialportion.

The manufacturing method of the platform of the optical transmitting andreceiving device of the present invention comprises the steps of forminga conduction layer by doping an impurity on the surface of thesubstrate, laminating an insulation layer on the surface of theconduction layer, providing an optical waveguide on the surface of theinsulation layer, and forming an electric wiring pattern on the surfaceof the insulation layer.

According to such an optical transmitting and receiving device of thepresent invention, since the current leaking from the LED to thesubstrate through the conduction layer can be eliminated, no currentleaks to the light receiving device side. Accordingly, the crosstalkbetween the transmitting side and the receiving side can be suppressed,the transmission side and the receiving side can be simultaneouslyoperated, and thereby a miniature hybrid-mounted optical transmittingand receiving device required to construct a subscriber system in anoptical communication network using an optical circuit can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Explains the configuration of a conventional optical transmittingand receiving device.

FIG. 2 explains the phenomenon that crosstalk is generated between anLED and a light receiving device in a hybrid-mounted opticaltransmitting and receiving device.

FIGS. 3A and 3B show an example of a hybrid-mounted optical device usinga PLC platform according to the present invention.

FIG. 4 explains the electrical crosstalk generating mechanism of anoptical transmitting and receiving device and the prevention method(No.1).

FIG. 5 explains the electrical crosstalk generating mechanism of anoptical transmitting and receiving device and the prevention method(No.2).

FIG. 6 explains one embodiment of the present invention and themanufacturing method (No.1).

FIG. 7 explains one embodiment of the present invention and themanufacturing method (No.2).

FIG. 8 explains one embodiment of the present invention and themanufacturing method (No.3).

FIG. 9 explains variant of one embodiment of the present invention andthe manufacturing method (No.1).

FIG. 10 explains variant of one embodiment of the present invention andthe manufacturing method (No.2).

FIG. 11 explains variant of one embodiment of the present invention andthe manufacturing method (No.3).

FIG. 12 explains one embodiment of the present invention and themanufacturing method (No.4).

FIG. 13 explains one embodiment of the present invention and themanufacturing method (No.5).

FIG. 14 explains one embodiment of the present invention and themanufacturing method (No.6).

FIG. 15 explains one embodiment of the present invention and themanufacturing method (No.7).

FIG. 16 explains one embodiment of the present invention and themanufacturing method (No.8).

FIG. 17 explains one embodiment of the present invention and themanufacturing method (No.9).

FIG. 18 explains one embodiment of the present invention and themanufacturing method (No.10).

FIG. 19 explains another embodiment of the present invention (No.1).

FIG. 20 explains variant of another embodiment of the present invention.

FIG. 21 explains another embodiment of the present invention (No.2).

FIG. 22 explains an embodiment in the case where an SiC or diamond layeris formed at the bottom of an LED mounting portion or a light receivingdevice mounting portion and the manufacturing method (No.1).

FIG. 23 explains an embodiment in the case where an SiC or diamond layeris formed at the bottom of an LED mounting portion or a light receivingdevice mounting portion and the manufacturing method (No.2).

FIG. 24 explains an embodiment in the case where an SiC or diamond layeris formed at the bottom of an LED mounting portion or a light receivingdevice mounting portion and the manufacturing method (No.3).

FIG. 25 explains an embodiment in the case where an SiC or diamond layeris formed at the bottom of an LED mounting portion or a light receivingdevice mounting portion and the manufacturing method (No.4).

FIG. 26 explains an embodiment in the case where an SiC or diamond layeris formed at the bottom of an LED mounting portion or a light receivingdevice mounting portion and the manufacturing method (No.5).

FIG. 27 explains an embodiment in the case where an SiC or diamond layeris formed at the bottom of an LED mounting portion or a light receivingdevice mounting portion and the manufacturing method (No.6).

FIG. 28 explains an embodiment in the case where an SiC or diamond layeris formed at the bottom of an LED mounting portion or a light receivingdevice mounting portion and the manufacturing method (No.7).

FIG. 29 explains an embodiment in the case where an SiC or diamond layeris formed at the bottom of an LED mounting portion or a light receivingdevice mounting portion and the manufacturing method (No.8).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 3A and 3B show an example of a hybrid-mounted optical device usinga PLC platform according to the present invention.

As shown in FIG. 3A, a platform substrate 11 is, for example, a siliconsubstrate, and an insulation film 18 is formed on the substrate. Theinsulation film 18 is made of SiO₂, etc. Furthermore, an LED 12, a lightreceiving device 16 and an overclad 19, excluding a transmitting unitwiring pattern 13 and a receiving unit wiring pattern 17, is provided onthe surface of this insulation layer 18. Accordingly, the insulationfilm 18 becomes the underclad of a waveguide 14. The overclad 19 is madeof SiO₂ or the same substance as the insulation film, such as lithiumniobate, etc. Then, a waveguide 14 for guiding light beams between theoverclad 19 and the underclad is formed. The waveguide 14 is structuredso as to guide light beams from the LED 12 to a WDM filter 15 providedin a groove in the overclad 19, and to guide optical signals transmittedand reflected by the WDM filter 15 to the light receiving device 16 anda light entrance/exit 20, respectively. The LED 12 is provided on theside of the WDM filter 15 where there is the entrance/exit 20 of thewaveguide 14, and the light receiving device 16 is provided on theopposite side of the WDM filter. The LED 12 is driven by the currentsupplied from the transmitting unit wiring pattern 13, and the lightreceiving device 16 outputs electric signals corresponding to receivedoptical signals to the receiving unit wiring pattern 17.

FIG. 3B shows the operation of the optical transmitting and receivingdevice shown in FIG. 3A.

The LED 12 transmits optical signals of a first wavelength (for example,1.3 μm) to the waveguide 14. The WDM filter 15 is designed so as toreflect the optical signals of the first wavelength. The optical signalsreflected by the WDM filter are outputted from the entrance/exit 20 ofthe waveguide 14.

On the other hand, optical signals of a second wavelength (for example,1.55 μm) different from the first wavelength are used for signalsreceived from the entrance/exit 20. The WDM filter 15 is designed totransmit optical signals of the second wavelength. Thus, the opticalsignals of the second wavelength received from the entrance/exit 20 passthrough the WDM filter, and are received by the light receiving device16. In this way, an optical transmitting and receiving devicehybrid-mounting a transmitting unit and a receiving unit can beconfigured by using different wavelengths for outgoing and incomingoptical signals.

In this configuration, the LED 12 is provided on the entrance/exit 20side of the WDM filter 15. This is because when outgoing optical signalsoutputted from the LED 12 or incoming optical signals received from theentrance/exit 20 leak out from the waveguide 14, and an oscillation modeis generated over all the overclad 19 and underclad (insulation layer18), this oscillation mode over all the overclad 19 and underclad isstray light which can be isolated by the WDM filter 15 to prevent thestray light from entering the light receiving device 16.

According to the configuration shown in this diagram, crosstalk betweenthe transmitting and receiving sides due to stray light can besuppressed to a minimum.

FIGS. 4 and 5 explain the electrical crosstalk generating mechanism of ahybrid optical transmitting and receiving device shown in FIGS. 3A and3B and the prevention method. FIG. 4 is the cross sectional diagram ofthe optical transmitting and receiving device, and shows that there isparasitic capacitance. FIG. 5 shows the equivalent circuit model.

As shown in FIG. 4, first, parasitic capacitances C₁₂ and C₃₄ aregenerated in the transmitting unit wiring pattern 13 and the receivingunit wiring pattern 17, respectively. Since the parasitic capacitancedoes not form a route from the LED 12 to the light receiving device 16as shown in the equivalent circuit of FIG. 5, this parasitic capacitancehas no effect on crosstalk between the transmitting side and thereceiving side. On the other hand, since the parasitic capacitanceC_(1S) and C_(2S) between the platform substrate 11 and the transmittingunit wiring pattern 13, and the parasitic capacitance C_(3S) and C_(4S)between the platform substrate 11 and the receiving unit wiring pattern17, form a route between the LED 12 and the light receiving device 16 asshown in the equivalent circuit of FIG. 5, this parasitic capacitancebecome a cause for generating crosstalk between the transmitting sideand the receiving side. Experimentally, it is found that the influenceof C_(1s) is greater than the influence of C_(2S), and the influence ofC_(3S) is greater than the influence of C_(4S) Since the impedance valueof the parasitic capacitance C_(SG) possessed by a silicon dioxide film21 plays a role of determining whether the current from the LED shouldflow to ground or to the light receiving device 16 as shown in theequivalent circuit of FIG. 5, the capacitance C_(SG) becomes animportant factor in the reduction of the electrical crosstalk.

Crosstalk is generated between the transmitting side and the receivingside since the driving current of the LED 12 leaks from the transmittingunit wiring pattern 13 to the receiving unit wiring pattern 17 viaseries capacitance (C_(1S), C_(2S), C_(3S) or C_(4S) in FIG. 4 or FIG.5) formed between the transmitting unit wiring pattern 13 and theplatform substrate 11 and between the receiving unit wiring pattern 17and the platform substrate 11. Since the branching ratio of the currentis determined by the ratio of the reciprocal of the impedance, it isnecessary to increase the impedance (1/jωC_(1S), 1/jωC_(2S), 1/jωC_(3S)and 1/jωC_(4S)) on the side connected to the receiving unit at eachbranching point and to reduce the impedance on the side not connected tothe receiving unit (1/j{acute over (ω)}C_(SG)) at each branching pointin order to reduce the current leaking to the receiving unit.

In the present invention, the impedance of the route for crosstalk isincreased by increasing the impedance between the transmitting unitwiring pattern 13 and the substrate 11, and the impedance between thereceiving unit wiring pattern 17 and the substrate 11. The crosstalkcurrent leaking to the substrate 11 is grounded by connecting thesubstrate 11 to a constant potential portion such as ground, etc.

Embodiments of the present invention and the manufacturing method aredescribed below with reference to FIGS. 6 through 18.

In these embodiments silicon is used for the substrate. The use of an Sisubstrate has advantages, such as being capable of accurately formingguiding grooves for optical fibers, ferrule, etc. using anisotropicetching, inexpensive material cost, etc.

As shown in FIG. 6, Ti (0.1 μm), Ni (0.1 μm) and Au (1μm) are vaporizedon the back of the Si substrate 301 as a back metallic film 302. This isneeded to electrically connect the Si substrate to a constant potentialportion such as ground, etc. and to make the substrate play a role inelectrostatic shielding. The back metallic film 302 may be vaporizedover all the back of the substrate, or only over the portions used forthe devices.

It is effective in reinforcing the shielding effect of the substrate todope a p type substrate and an n type substrate with a high density ptype impurity and a high density n type impurity, respectively, andforming an ohmic contact prior to the vaporization of the back metallicfilm 302. At this moment, since a thin silicon dioxide film is formed atthe back of the substrate 301 as described before, it is recommended toremove the silicon dioxide film by cleaning the back of the substrate301 using fluoric acid, etc. Then, the above-mentioned back metallicfilm 302 is formed before a new silicon dioxide film is generated. Sinceby grounding the back metallic film 302 the whole substrate can be keptat a constant potential, and the current leaking from the LD can beeliminated, the flow of leakage current to the PD can be suppressed, andthereby the crosstalk can be reduced.

Then, as shown in FIG. 7, resist 303 is applied to optical devicemounting portions 305 and 306 on the surface of the Si substrate 301using a photo lithography technology, and the other portions of thesurface of the Si substrate 301 are etched approximately 20 μm deepusing a KOH solution, etc. In this case, the metallic film is preventedfrom being etched by using a resist 304. Thus, convex portions 305 and306 are formed at the optical device mounting portions of the Sisubstrate.

In the process shown in FIG. 8, after the resist 303 and 304 areremoved, an n type impurity such as boric acid, etc. and a p typeimpurity such as phosphorus, etc. are doped on the surface of thesubstrate 301, for example, using a vapor diffusion method, etc. to forma conduction layer 307.

When the conduction layer 307 is formed over all the surface of thesubstrate as shown in FIG. 8, there is no need for a mask layer.

In another method to form optical device mounting portions, as shown inFIG. 9, SiO₂ layer 303 is applied to optical device mounting portions305 and 306 on the surface of the Si substrate 301 using a photolithography technology, and the other portions of the surface of the Sisubstrate 301 are etched approximately 20 μm deep using a KOH solution,etc. Thus, convex portions 305 and 306 are formed at the optical devicemounting portions of the Si substrate.

Then, as shown in FIG. 10, Ti (0.1 μm), Ni (0.1 μm) and Au (1 μm) arevaporized on the back of the Si substrate 301 as a back metallic film302. This is needed to electrically connect the Si substrate to aconstant potential portion such as ground, etc. and to make thesubstrate play a role in electrostatic shielding. The back metallic film302 may be vaporized over all the back of the substrate, or only overthe portions used for the devices.

It is effective in reinforcing the shielding effect of the substrate todope a p type substrate and an n type substrate with a high density ptype impurity and a high density n type impurity, respectively, andforming an ohmic contact prior to the vaporization of the back metallicfilm 302. At this moment, since a thin silicon dioxide film is formed atthe back of the substrate 301 as described before, it is recommended toremove the silicon dioxide film by cleaning the back of the substrate301 using fluoric acid, etc. Then, the above-mentioned back metallicfilm 302 is formed before a new silicon dioxide film is generated. Sinceby grounding the back metallic film 302 the whole substrate can be keptat a constant potential, and the current leaking from the LD can beeliminated, the flow of leakage current to the PD can be suppressed, andthereby the crosstalk can be reduced.

In the process shown in FIG. 11, after the resist 303 and 304 areremoved, an n type impurity such as boric acid, etc. and a p typeimpurity such as phosphorus, etc. are doped on the surface of thesubstrate 301, for example, using a vapor diffusion method, etc. to forma conduction layer 307.

When the conduction layer 307 is formed over all the surface of thesubstrate as shown in FIG. 11, there is no need for a mask layer.

Then, as shown in FIG. 12, an SiO₂ layer 308 over 30 μm thick isdeposited using a flame deposition method, etc., and the surface of theSiO₂ layer 308 is leveled by polishing. At this time, an SiO₂ film 1 μmthick is left above the convex portions 305 and 306 for mounting theoptical devices, particularly on the LED mounting portion 305 whichgenerates a lot of heat.

Alternatively, in the polishing process the substrate can be polisheduntil the Si substrate of the optical device mounting portions 305 and306 is exposed, and after that a new SiO₂ film approximately 1 μm thickcan be formed again.

The SiO₂ layer 308 formed in the process shown in FIG. 12 becomes theunderclad of the optical waveguide and the insulation layer between theelectric wiring patterns and the Si substrate.

Then, in the process shown in FIG. 13, a core 309 of the waveguide andan overclad 310 are formed in succession, and the waveguide ends 309 aare formed on the optical device mounting portion sides.

An SiO₂ layer doped with Ge, Ti, etc. is laminated approximately 8 μmthick, portions to be left as a core 309 are covered with resist (notshown in the diagram), and the SiO₂ layer is etched using RIE (ReactiveIon Etching) with this resist as a mask until the underclad 308 isexposed.

Further, an SiO₂ layer is laminated approximately 30 μm thick, andportions other than the optical device mounting portions 305 and 306 arecovered with resist 311. The SiO₂ layer to be the optical devicemounting portions is etched using RIE until the SiO₂ layer on the Sisubstrate convex portions of the optical device mounting portions 305and 306 formed in the process shown in FIG. 12 becomes approximately 1μm thick. Alternatively, after etching the layer until the Si (silicon)of the Si substrate convex portions of the optical device mountingportions 305 and 306 is exposed, a heat-oxidized film approximately 1 μmthick can be formed.

At this time, contact holes 323 to the conduction layer 307 are providedin the optical device mounting portions.

Next, as shown in FIG. 14, electric wiring patterns 312 are formed usinga lift-off method.

First, resist 313 is applied over all the surface of the substrateapproximately 2 μm thick (the resist must be thicker than the electricwiring patterns 314 to be formed), and the resist 313 of portions to bepatterned is removed. Then, for example, Ti (0.1 μm), Ni (0.1 μm) and Au(1 μm) 314 are vaporized over the surface of the substrate in thatorder, and surplus vaporized metallic films are removed together withthe resist 313.

Solder bumps made of gold, tin, etc. can also be formed in the opticaldevice mounting portions using a lift-off method in the same way asdescribed above, if necessary.

Aligning markers 316 can also be formed in the optical device mountingportions simultaneously with the electric wiring patterns 314, andoptical elements can be aligned by referring to these markers whenmounting the devices.

In the process shown in FIG. 15 a groove 317 for inserting a multilayerfilm filter is formed using a dicing saw, and the Si substrate 301 iscut into individual platforms. The groove 317 is formed so as to crossthe point where an optical waveguide is branched, and the depth is equalto or greater than the depth of the provided underclad.

In the process shown in FIG. 16, optical devices 318 and 319 are alignedby referring to the aligning markers formed in the optical devicemounting portions, as described above, and the optical devices 318 and319 are installed onto the electric wiring patterns 312 by heating theplatform. A multilayer film filter 320 is inserted, and is fixed using alight-transmitting adhesive 321. In this embodiment, first, the opticaldevices 318 and 319 are mounted using a AuSn solder bumps 315 (FIG. 14)formed on the electric wiring patterns 312, and then the light receivingdevice 319 is mounted using a conductive adhesive. After the opticaldevices are mounted, the optical devices and the electric wiringpatterns are connected with each other using wires 322. Then, conductionlayer pads 324 are provided. The conduction layer pads 324 areelectrically connected to the conduction layer 307 through the contactholes 323, and the pads 324 are further connected to the terminals ofthe package later.

In the process shown in FIG. 17, a platform 326 is fixed to a package325, the electric wiring patterns 312 of the platform, the conductionlayer pads 324 (FIG. 16) and the terminals 327 of the package 325 areconnected with each other using Au wires 329. The metallic film 302 atthe back of the substrate and the radiation pads (at the bottom of theplatform 326) are stuck together using a conductive adhesive such assilver paste, etc.

In the process shown in FIG. 18, the cores of optical fiber 331 arealigned to the optical waveguide, and the optical fibers are connectedusing a thermosetting adhesive, UV-hardening adhesive, etc. After theoptical fiber is connected to the optical waveguide using a fiberconnecting material 330, the platform is encapsulated using a resin 332,etc.

In the optical transmitting and receiving device described above withreference to FIGS. 6 through 18, since the conduction layer 307 isformed on the substrate 301, and this conduction layer 307 is groundedthrough the contact hole 323, the currents leaking from the LED 318 aregrounded, and the crosstalk to the light receiving device 319 can bereduced.

The electrical crosstalk can also be reduced by improving the insulationstate between the electric wiring pattern 312 connected to the LED 318or the light receiving device 319, etc. and the substrate 301, byleaving the SiO₂ layer underneath the LED 318 and the light receivingdevice 319 rather thick and forming a heat-oxidized film.

As described above, according to this embodiment, since the electricalcrosstalk can be suppressed, a more reliable hybrid-mounted opticaltransmitting and receiving device can be provided.

FIGS. 19 and 21 explain another embodiment of the present invention.

If the Si substrate 46 including the light receiving device mountingportion 45 is etched with resist 42 leaving only the LED mountingportion 41, as shown in FIG. 19, a thick insulation layer can be formedat the bottom of the light receiving device mounting portion 45 in theprocess shown in FIG. 12.

Thus, since the insulation layer at the bottom of the light receivingdevice is thick, thereby more effective insulation can be obtained.

A back protection resist 44, provided so as to cover the back metallicfilm 43 shown in FIG. 19, is provided to prevent the back metallic film43 from being etched when etching the Si substrate 46. For etching, forexample, the above-mentioned KOH is used.

If the Si substrate 46 including the light receiving device mountingportion 45 is etched with SiO₂ 42 leaving only the LED mounting portion41, as shown in FIG. 20, a thick insulation layer can be formed at thebottom of the light receiving device mounting portion 45 in the processshown in FIG. 12.

Thus, since the insulation layer at the bottom of the light receivingdevice is thick, thereby more effective insulation can be obtained.

After removing the SiO₂ mask with HF, the back metallic film isvaporized on the back of the substrate.

Although in the embodiment of FIGS. 6 to 18 a conduction layer is formedover all the surface of the substrate 56, as shown in the process shownin FIG. 8, as shown in FIG. 21, there is no need to form a conductionlayer over all the surface of the substrate 56 when a contact hole forgrounding is provided in the light receiving device mounting portion 55and the LED mounting portion 54. In this case, it is sufficient if theconduction layer is formed only in the light receiving device mountingportion 55 and the LED mounting portion 54.

For this reason, in the embodiment shown in FIG. 21, a conduction layer51 is selectively formed only on the LED mounting portion 54 and thelight receiving device mounting portion 55. When the substrate 56 is ptype Si, the conduction layer is formed, for example, by doping thesubstrate 56 with boron, etc. When the substrate 56 is n type Si, theconduction layer is formed, for example, by doping the substrate 56 withphosphorus, etc.

To selectively dope only the LED mounting portion 54 or the lightreceiving device mounting portion 55, an impurity is doped after a masklayer 52 made of SiO₂ is formed using a photo lithography technology.That is, doping is performed using boron or phosphorus.

A pn junction can be formed at the optical device mounting portions 54and 55 by doping the substrate 56 with an impurity the reverse of theconduction type of the substrate 56. That is, if the substrate 56 is ptype Si, the conduction type of both the LED mounting portion 54 and thelight receiving device mounting portion 55 is made n type. When thevoltage generated in the LED mounting portion 54 and the light receivingdevice mounting portion 55 is higher than ground, this configuration iseffective. Because by making the conduction type of both the LEDmounting portion 54 and the light receiving device mounting portion 55 ntype, the pn junction is reversely biased by the voltage applied betweenthe back of the substrate 56 and the optical device mounting portions,and it becomes difficult for current to flow through the pn junction.Conversely, when the voltage generated in the LED mounting portion 54and the light receiving device mounting portion 55 is lower than ground,n type Si is used for the substrate 56, and the conduction type of boththe LED mounting portion 54 and the light receiving device mountingportion 55 is made p type. Thus, since, as described above, the pnjunction is reverse-biased by the voltage applied between the back ofthe substrate 56, and the LED mounting portion 54 and the lightreceiving device mounting portion 55, the current flowing through thispn junction can be eliminated. Accordingly, the current from the LEDprovided in the LED mounting portion 54 can be prevented from flowing ascrosstalk into the light receiving device provided in the lightreceiving device mounting portion 55 through the substrate 56. In thisway, since the crosstalk can be prevented by applying a reverse bias tothe pn junction, there is no need of a configuration for connecting theconduction layer 307 to a constant potential, as required in theprevious embodiment.

Alternatively, both sides of the substrate can also be doped in theprocess shown in FIG. 7. In this case, the process can also be arrangedso that the conduction layer is left only on the convex surface of theoptical device mounting portions on the surface of the substrate in theetching process shown in FIG. 7.

As described above, a crosstalk route is made to have a high impedanceby utilizing the inverse direction characteristic of the pn junction orincreasing the resistance at the bottom of the optical device mountingportions of the substrate. Alternatively, signals of the LED leaked tothe substrate are grounded, etc. through a low resistance layer(conduction layer) formed beneath the insulation film 18 shown in FIG.3A. As a result, the crosstalk from the LED to the light receivingdevice can be reduced.

According to the above-mentioned embodiments, low crosstalk can berealized in a miniature optical transmitting and receiving devicehybrid-mounting an LED and a light receiving device, and thereby aminiature optical transmitting and receiving device for an ATM-PON canbe implemented.

A layer made of a plastic waveguide material such as polyimide, etc. canalso be formed instead of the SiO₂ layer formed above. Using aninsulating material such as heat-conductive SiC, diamond, etc. has anadvantage that the heat radiation of an LED generating a lot of heat canbe secured even if a thick insulation layer is used to reduce theparasitic capacitance of the LED mounting electrodes.

FIGS. 22 through 29 show an embodiment in the case where SiC or adiamond layer are formed at the bottom of an LED mounting portion or alight receiving device mounting portion and the manufacturing method.

First, as shown in FIG. 22, a dopant is applied to the top of an Sisubstrate 800, and a conduction layer 801 is formed. Then, as shown inFIG. 23, a diamond or SiC layer 802 is formed on the conduction layer801. Then, as shown in FIG. 24, resist 803 is applied to the layer 802except for the LED mounting portion, and Si is selectively grown. Atthis time, the Si does not have to be a single crystal, but can bepolycrystalline. Thus, a Si base is formed at the LED mounting portion.This base is used later to form a waveguide, and is also used to alignthe LED since high accuracy is required in the alignment of the LED. Onthe other hand, in the case of the light receiving device, such a baseis not required since such high accuracy is not required. As a result,the insulation layer beneath the light receiving device becomes thick,and thereby effective insulation can be obtained. After a base for theLED mounting portion is formed, the resist 803 is removed in the processshown in FIG. 24.

Then, as shown in FIG. 25, a SiO₂ layer 805 to be used for an insulationlayer is formed using a flame deposition method, etc., and as shown inFIG. 26, the top of the SiO₂ layer 805 is polished and leveled until abase for the LED mounting portion appears.

Then, as shown in FIG. 27, another SiO₂ layer 806 is laminated. Thislayer later becomes an underclad. A layer 807 to be used for a waveguideof which the refractive index is greater than the refractive index ofthe SiO₂ layer 806 is laminated on this. The waveguide is formed bylaminating a substance to be used for a waveguide over all the surfaceof the SiO₂ layer 806, and then etching the layer after applying resistto a portion to be used for a waveguide. Another SiO₂ layer 808 isfurther laminated on the waveguide formed in this manner. This laterbecomes an overclad.

Then, as shown in FIG. 28, portions to be used for a waveguide are leftas they are, and portions where the LED LD and the light receivingdevice PD are to be provided later are etched. The etching shall be sodeep that the top of the base for the LED mounting portion may appear.Then, the top of the base for mounting the LED LD is oxidized by heatingin an atmosphere of oxygen and steam, and electrodes 812 are formed onand adjacent to the top of the base. On the light receiving side,electrodes 811 are provided on the SiO₂ layer 805. As shown in FIGS. 3Aand 3B, a groove is formed using a dicing saw 813 in a portion where awaveguide is branched in order to insert a multilayer film filter (WDMfilter).

Finally, as shown in FIG. 29, a multilayer film filter 817 is insertedin the groove formed using the dicing saw 813 and fixed using a lighttransmitting adhesive, and the LED 814 and light receiving device 815are mounted and aligned.

Although a metallic film is formed at the back of the substrate 800 asdescribed above, the metallic film is omitted in these diagrams. Then,the potential of the substrate 800 is set to ground potential bygrounding the metallic film.

In the above-mentioned embodiments, since a diamond layer or SiC layerwith a low conductivity is provided, current can be more effectivelyprevented from flows from the LED 814 into the substrate 800.Furthermore, since the diamond layer or SiC layer has a highheat-conductivity, the heat of the LED 814 can be easily radiated, andthe heat can be prevented from being concentrated. Since the bottom ofthe LED 814 is heat-oxidized, the insulation is further improved.

Furthermore, the bottom of the light receiving device 815 is anunderclad 810, which is used as an insulation film. Particularly, sincethere is no base at the bottom of the light receiving device 815, andthe insulation layer is thick, the inflow of current from the lighttransmitting element 814 can be effectively prevented.

Although in the previous embodiments the conduction layer 801 isdirectly grounded, in this embodiment the conduction layer 801 is notdirectly grounded. This is because the conduction layer 801 issubstantially grounded by grounding the back of the substrate 800, sinceit is difficult to provide a diamond layer or SiC layer with a contacthole. Since the substrate 800 is made of Si, the diamond layer or SiClayer has a high electrical conductivity, and the current easily flowsfrom the conduction layer 801 to the back side of the substrate 800,even if only the back side of the substrate 800 is grounded.Accordingly, almost the same effect as the case where the conductionlayer 801 is directly grounded can be obtained.

As described above, according to the present invention, a means forreducing the crosstalk between the hybrid-mounted LED and lightreceiving device can be provided. Accordingly, a compact optical devicecapable of simultaneously transmitting and receiving can be provided,and greatly contributes to the spread of optical subscriber systems.

What is claimed is:
 1. A platform of an optical transmitting andreceiving device, comprising: a conduction layer formed at the top or apart of a substrate; an insulation layer formed below at least a lightemitting device mounting portion and light receiving device mountingportion; an optical waveguide formed on the insulation layer; anelectric wiring pattern formed on the insulation layer; and wherein saidconduction layer can be electrically connected to a constant potentialportion, and said insulation layer includes an SIC layer or diamond thinfilm layer.
 2. An optical transmitting and receiving devicehybrid-mounting at least a light emitting device and a light receivingdevice on the same substrate through an insulation layer, wherein aconduction layer is interposed at least below said light emitting deviceand said light receiving device and between said substrate and saidinsulation layer, and the conduction layer is electrically connected toa constant potential portion and the substrate is insulated from thelight emitting device and the light receiving device with highimpedance, and said insulation layer at the bottom of the lightreceiving device is thicker than said insulation layer at the bottom ofthe light emitting device.
 3. An optical transmitting and receivingdevice according to claim 2 wherein said substrate is a semiconductor ofone conduction type, a semiconductor layer of another conduction typethe reverse of said one conduction type which forms a pn junctiontogether with said semiconductor substrate is interposed at least belowsaid light emitting device and said light receiving device and betweensaid substrate and said insulation layer, and voltage applied betweensaid light emitting device and said light receiving device, and the backof said semiconductor substrate is reverse-biased to said pn junction.4. An optical transmitting and receiving device, comprising: aconduction layer formed on all or a part of a top of a substrate; aninsulation layer formed at a bottom of at least a light emitting devicemounting portion and a light receiving device mounting portion on thesubstrate; an optical waveguide formed on the insulation layer; anelectric wiring pattern formed on the insulation layer; and a lightemitting device and a light receiving device connected to the electricwiring pattern so as to be optically coupled with the waveguide, whereinsaid conduction layer is electrically connected to a constant potentialportion and is insulated from the electric wiring pattern with highimpedance, and said insulation layer includes an SiC layer or diamondthin film layer.
 5. An optical transmitting and receiving device,comprising: a conduction layer formed on all or a part of a top of asubstrate; an insulation layer formed at a bottom of at least a lightemitting device mounting portion and a light receiving device mountingportion on the substrate; an optical waveguide formed on the insulationlayer; an electric wiring pattern formed on the insulation layer; and alight emitting device and a light receiving device connected to theelectric wiring pattern so as to be optically coupled with thewaveguide, wherein said conduction layer is electrically connected to aconstant potential portion and is insulated from the electric wiringpattern with high impedance, and said insulation layer at the bottom ofthe light receiving device is thicker than said insulation layer at thebottom of the light emitting device.
 6. The optical transmitting andreceiving device according to claim 4, wherein said conduction layer isgrounded.
 7. The optical transmitting and receiving device according toclaim 4, wherein said substrate is a semiconductor substrate, and saidconduction layer is a low resistance layer doped with a p type or n typeimpurity on a surface of the semiconductor substrate.
 8. The opticaltransmitting and receiving device according to claim 7, wherein a Sisubstrate is used for said semiconductor substrate.
 9. The opticaltransmitting and receiving device according to claim 7, wherein a wholesurface of said semiconductor substrate is doped with an impurity. 10.The optical transmitting and receiving device according to claim 7,wherein the bottom of a light emitting device mounting portion or alight receiving device mounting portion of said substrate is doped withan impurity.
 11. The optical transmitting and receiving device accordingto claim 4, wherein said substrate is an n type semiconductor substrate,the bottom of the light emitting device mounting portion or the lightreceiving device mounting portion of the n type semiconductor substrateis doped with a p type impurity, and a potential of the wiring patternon the insulation layer is lower than a potential of the substrate. 12.The optical transmitting and receiving device according to claim 4,wherein said substrate is a p type semiconductor substrate, the bottomof the light emitting device mounting portion or the light receivingdevice mounting portion of the p type semiconductor substrate is dopedwith an n type impurity, and a potential of the wiring pattern on theinsulation layer is higher than a potential of the substrate.
 13. Anoptical transmitting and receiving device, comprising: a conductionlayer formed on all or a part of a top of a substrate; an insulationlayer formed at a bottom of at least a light emitting device mountingportion and a light receiving device mounting portion; an opticalwaveguide formed on the insulation layer; an electric wiring patternformed on the insulation layer; and a light emitting device and a lightreceiving device connected to the electric wiring pattern so as to beoptically coupled with the waveguide, wherein said conduction layer iselectrically connected to a constant potential portion, and saidinsulation layer includes an SiC layer or diamond thin film layer. 14.An optical transmitting and receiving device, comprising: a conductionlayer formed on all or a part of a top of a substrate; an insulationlayer formed at a bottom of at least a light emitting device mountingportion and a light receiving device mounting portion; an opticalwaveguide formed on the insulation layer; an electric wiring patternformed on the insulation layer; and a light emitting device and a lightreceiving device connected to the electric wiring pattern so as to beoptically coupled with the waveguide, wherein said conduction layer iselectrically connected to a constant potential portion, and saidinsulation layer at the bottom of the light receiving device is thickerthan said insulation layer at the bottom of the light emitting device.15. An optical transmitting and receiving device according to claim 13,wherein said substrate is a semiconductor substrate, and said conductionlayer is a low resistance layer doped with a p type or n type impurityon the surface of the semiconductor substrate.
 16. The opticaltransmitting and receiving device according to claim 15, wherein a Sisubstrate is used for said semiconductor substrate.
 17. The opticaltransmitting and receiving device according to claim 15, wherein a wholesurface of said semiconductor substrate is doped with an impurity. 18.The optical transmitting and receiving device according to claim 15,wherein the bottom of a light emitting device mounting portion or alight receiving device mounting portion of said substrate is doped withan impurity.
 19. An optical transmitting and receiving device accordingto claim 13; wherein said substrate is an n type semiconductorsubstrate, the bottom of the light emitting device mounting portion orthe light receiving device mounting portion of the n type semiconductorsubstrate is doped with a p type impurity, and a potential of the wiringpattern on the insulation layer is lower than a potential of thesubstrate.
 20. An optical transmitting and receiving device according toclaim 13, wherein said substrate is a p type semiconductor substrate,the bottom of the light emitting device mounting portion or the lightreceiving device mounting portion of the p type semiconductor substrateis doped with an n type impurity, and a potential of the wiring patternon the insulation layer is higher than a potential of the substrate. 21.The optical transmitting and receiving device according to claim 4,further comprising filter means provided to intercept said opticalwaveguide, for reflecting optical signals outputted from said lightemitting device and transmitting optical signals to be received by saidlight receiving device, wherein the light receiving device and the lightemitting device are located on each side of the filter means, and theexit of optical signals from said optical transmitting and receivingdevice is provided on the location side of the light emitting device.